1. Field of the Invention
The invention relates to a process for the electrolytic application of a thin silver coating of a thickness between 0.8 and 1.8 microns to stainless steel parts of the "bearing steel" type, for example Z100 CD 17 or E80CDV40 (AFNOR standard), i.e. very low deformation steels of hardness greater than 58HRc.
2. Summary of the Prior Art
In the silverplating of jewellery items it is known to apply the silverplating as a thick layer preceded by nickelplating. French Pat. No. 1,442,506 describes a process of this type. The conditions described for carrying out this process are not suited to the substrates to which the present invention relates. For example they specify preliminary nickel plating at a temperature from 40.degree. to 60.degree. C., a temperature at which bearing steels, which are very sensitive to high temperatures in hydrochloric surroundings, would experience the dissolution of their carbides. In addition, the implementation of the silverplating steps involves a preliminary silverplating under conditions such that the thickness of the pre-silverplating layer will have a tolerance of from 1 to 5 microns, whereas the thickness of the silverplating coating will be 30 microns.
Such a process is absolutely unsuitable for solving the problem addressed by the invention, which is to provide a very adherent silver coating on a bearing steel in a reproducible manner, the coating having a substantially uniform thickness not exceeding 1.8 microns.
In some applications of ball or roller bearings it is required that the bearings should be lubricated for life and not call upon external lubricants such as oils or greases, either because overall size constraints prevent the setting up of lubricating means, or because the environment does not allow the use of lubricating greases or oils because of the danger of contamination of the lubricant by the environment, or of the risks that the lubricant might create (for example when the bearings are used in atmospheres where a heated lubricant could cause a fire or an explosion).
In these applications, the avoidance of external lubricants has led to the provision of the races of roller or ball bearings with a coating layer having a very low friction coefficient, such as a coating including graphite or molybdenum bisulphide.
However, it has been found that such coatings, if applied directly to bearing steels, do not have a long service life because of their very low adherence to said steels. It has also been found that this adherence can be improved if the support is subjected to silver plating preliminarily to the deposition of the lubricating coating.
Thus, a standard silvering operation before the deposition of the lubricating coating comprises subjecting the parts to the following steps:
degreasing in a solvent bath; PA1 electrolytic anodic degreasing in an alkaline atmosphere PA1 electrolytic attack in an acid atmosphere; PA1 chemical neutralization in a sodium carbonate bath, PA1 silvering in an electrolytic bath of silver cyanide (1.2 g/l), potassium cyanide (37.5 g/l), sodium carbonate (22.5 g/l) and soda (37.5 g/l) for one minute at ambient temperature and a current density between 5 and 10 A/dm.sup.2 ; PA1 rinsing; PA1 drying in air; and, PA1 degassing in air at 130.degree. C. for 2 hours. PA1 (a) degreasing in a solvent bath; PA1 (b) anodic electrolytic degreasing in an alkaline atmosphere; PA1 (c) anodic electrolytic attack in an acid atmosphere; PA1 (d) an intermediate step of additional preparation; PA1 (e) electrolytic neutralization; PA1 (f) electrolytic presilvering in a presilvering bath containing silver cyanide (Ag CN), potassium cyanide (KCN), and potassium carbonate (K.sub.2 CO.sub.3) in proportions such that the free KCN/metal Ag ratio is greater than 15; and, PA1 (g) electrolytic silvering proper in a silvering bath containing silver cyanide, potassium cyanide, potassium carbonate, and potash in proportions such that the free KCN/metal Ag ratio of the silvering bath is about 10 times lower than that of the presilvering bath.
It has been observed, however, that ball bearings which have undergone such a treatment before the lubricating coating, if subjected to high stresses such as under substantial and rapid variation of rotational speed, or also sudden heating, tend to seize very quickly and thereby bring about the deterioration of the device in which they are incorporated. Analyses carried out on such seized bearings have identified various types of problems.
It was first of all realised that the silvering process such as described above does not enable a sufficiently substantial thickness of silver to be obtained.
Indeed, electrolysis at a high current density (above 5A/dm.sup.2) with the silvering bath composition referred to above brings about a cathodic polarization (due to the sodium ions at high current density) which quickly becomes substantial and blocks the electrolytic silver deposition at a thickness which does not exceed 0.5 micron.
There was also observed, during silvering, a pronounced gas release which creates a risk of fatigue of the steel substrate due to hydrogen, a risk which is all the more significant as the silver thickness is restricted to 0.5 micron and does not provide an adequate barrier for preventing the diffusion of hydrogen into the substrate.
Moreover, air degassing at 150.degree. C. of silver deposits of 0.5 micron causes unreliable adherence of the silver to the substrate, as evidenced by magnification analyses showing an oxidation of the silver deposit and points on the under-layer inducing a loss of adhesion.
The Chemical Abstract "Treatment of a Stainless Steel Strip Before Plating" from Furukawa Electric Co. (Vol. 102, 1985-211 592 g) describes effecting a deposition of silver to a thickness of 1 micron on ferritic stainless steel sheets by plating nickel in a hydrochloric atmosphere, then pre-silvering at 3 A/dm.sup.2 for 10 seconds, and finally silvering at 0.5 A/dm.sup.2 for 3 minutes. Such a process is not easily usable on an industrial scale for the purpose of the present invention, which is to make a layer of very small thickness (below 1.8 microns) in a reproducible manner. A pre-silvering time of 10 seconds is difficult to reproduce industrially in a regular manner since it is such a short period, and the slightest time variation brings about a substantial variation of the pre-silvering thickness by virtue of the high current density used. In addition, with such a current density, the silver underlayer will be rough and can only lead to a poor final silvering result.
It is therefore an object of the invention to provide an industrial silvering process which avoids these drawbacks and makes it possible to obtain, reproducibly, silver deposits of a thickness between 0.8 and 1.8 microns, which are substantially uniform over the surface to be covered, and are very adherent and do not lead to hydrogen fatigue or loss of ductility of the coated part.
It is also an object of the invention to facilitate the production of an adherent and durable deposit of an anti-friction material such as molybdenum bisulphide or graphite on ball or roller bearing races by first providing the race with a suitable silver coating.